This invention relates to alumina abrasive materials and specifically to alumina made by a sol-gel process. In such processes a sol or gel of a precursor of alpha alumina is formed and then dried and fired to the alpha form. The selected precursor is most frequently boehmite but earlier precursors such as alumina trihydrate can be used. One advantage of such aluminas in that they do not use a fusion process and are therefore much more efficient in energy usage. They are also characterized by crystal sizes in the range of a few microns and this seems to be associated with good grinding properties when compared to the relatively large crystal chips obtained by crushing the product of the fusion process.
These sol-gel processes are now well-known in the art having been the subject of great interest since the early '80s when the first sol-gel alumina abrasive grits were developed. The development was given a great boost in the mid-80s with the introduction of the vastly superior seeded sol-gel aluminas in which the sol-gel was seeded with a substance capable of lowering the temperature at which the conversion to alpha alumina occurs. It is generally accepted that this operates by a mechanism involving epitaxial growth of alpha alumina on the surface of the seed which therefore needs to be of the same crystallographic type as alpha alumina and with similar lattice parameters within the crystals. The result is a very fine, uniform sub-micron crystalline structure that seems to be associated with good abrasive performance. When reference is made to a sol-gel alumina hereinafter, it is to be understood that this is intended to cover all processes of the above type and their obvious variants that result in aluminous abrasive grains with a high density, small crystal size, (below about 10 microns for example), and high hardness, (greater than about 16 Gpa for example).
Other ways of obtaining smaller crystalline structures within the sol-gel alumina art includes the incorporation of cell growth control agents which can restrain crystal growth such that quite uniform structures with crystal sizes ranging from just over 1 to about 10 microns depending on the process and the agent used. Such additives in general do not reduce the transition temperature at which alpha alumina is formed indeed some, such as silica, can actually increase it. They can however introduce interesting properties. Such modification seems to be associated with modified fracture mechanics which in some applications can be advantageous.
There is however a tendency for the modifiers to be concentrated at the surfaces of the abrasive grits and this means that any beneficial effect associated with the presence of the modifiers can be expected to be inconstant.
The present invention however provides aluminous abrasive grits in which the concentration of modifying components is essentially constant across the full thickness of the abrasive grit and a novel process by which such modified aluminous abrasive grits can be made.